Tek-mdo3104-360_01a_I2C.jpg

Dave Young of Young Circuit Designs is publishing a series of articles and videos on element14 every other week to describe his experience in upgrading from his trusty Tektronix TDS2024B to a new Tektronix MDO3104. See his other articles in the series:

(a) Tek MDO3104 Scope Upgrade Series #1: The 70% Test with a Tek MDO3000

(b) Tek MDO3104 Scope Upgrade Series #2: Digital Decoding and Triggering for Analog Analysis

(c) Tek MDO3104 Scope Upgrade Series #3: Trace Persist and Analog Triggering

(d) Tek MDO3104 Scope Upgrade Series #4: Computer Connectivity and Remote Control

Despite all of the features manufacturers now include in popular mixed-domain scopes, its primary function is to examine and characterize analog signals. But how can you examine a signal that you're not sure exists? This article and video looks at my existing Tek TDS2024B and its replacement, the Tek MDO3104Tek MDO3104, in order to compare how effective they are at finding and examining a 'runt' signal.

 

Engineers everywhere know the sneaking feeling that something about a waveform isn't right. It's hard to characterize; it could look like a fleeting glitch on the scope trace, or an occasional failure that behaves a lot like loose cables. But it causes enough pause for the engineer to take a closer look.

A 'runt' signal is one that wants to be a pulse, but doesn't make it all the way up to the target 'High' voltage for the duration of the pulse. Sometimes these can occur only seldomly in a long pulse train, making it difficult to identify and trigger. For this review, I will look at a runt signal that occurs every second or so.

The first feature that both scopes share is 'Persist' where instead of displaying only one trace, the scope will continually lay each new trace over old ones. With this, one should be able see a trace that doesn't fall within the norm, even if it rarely occurs.

 

To access the persist feature on the TDS2024B, one can press

Horiz. Menu > Persist ON softkey > Adjust persist time to infinite

For the runt signal examined in this case, the 2024B did not catch the runt signal, even after allowing it to collect data for several minutes. I know from using the 3104 that a runt exists, but disappointingly the 2024B falls short with this feature. It simply didn't catch the runt.

 

To access the persist feature on the MDO3104, one can press

Acquire > Enable Fast Acq Softkey > Waveform Display Softkey > Persist On softkey > Adjust Persist Time to Infinite.

The runt signal that the 2024B missed comes up quickly on the MDO, and the color shading is a nice feature add to give an understanding of how often a trace occurs.

 

From there, the MDO has additional triggering capability to allow us to see the troublesome trace more clearly. In 'Runt' mode, one can set a high and low threshold such that a trigger will only occur when an edge exceeds one limit, but not the other. This is in contrast to the standard edge trigger offered by the 2024 which only has one threshold. To setup a runt trigger:

Trigger Menu > Knob 'A' to 'Runt' > Set Thresholds with knobs 'A' and 'B'

 

This is the first time that my trusty TDS2024B had a similar feature to the MDO3104 but it wasn't able to compete. The extra trigger functionality was a welcome addition, since even if the TDS could have captured it there wouldn't be a good way to get a closer look at it with a more complicated trigger. This would have been one case where upgrading the MDO would have allowed me to capture a problem that I would have missed with the TDS scope, which makes the upgrade totally worth it!

Dave Young of Young Circuit Designs is publishing a series of articles and videos on element14 every other week to describe his experience in upgrading from his trusty Tektronix TDS2024B to a new Tektronix MDO3104. See his other articles in the series:

(a) Tek MDO3104 Scope Upgrade Series #1: The 70% Test with a Tek MDO3000

(b) Tek MDO3104 Scope Upgrade Series #2: Digital Decoding and Triggering for Analog Analysis

(c) Tek MDO3104 Scope Upgrade Series #3: Trace Persist and Analog Triggering

(d) Tek MDO3104 Scope Upgrade Series #4: Computer Connectivity and Remote Control